An XAS study of the binding and reduction of Au(III) by hop biomass

Microchemical Journal 81 (2005) 50 – 56 www.elsevier.com/locate/microc

An XAS study of the binding and reduction of Au(III) by hop biomass M.L. Lo´peza, J.G. Parsonsb,T, J.R. Peralta Videab, J.L. Gardea-Torresdeya,b a

Environmental Science and Engineering PhD Program, University of Texas at El Paso, 500 W. University Ave. El Paso, TX 79968, United States b Department of Chemistry, University of Texas at El Paso, 500 W. University Ave. El Paso, TX 79968, United States Received 22 January 2004; accepted 21 January 2005 Available online 17 March 2005

Abstract Previous studies have shown that hop biomass is capable of adsorbing significant amounts of Au(III) from aqueous solutions. Hop biomass was chemically modified to determine the contributions that the different functional groups on the biomass have on the binding and reduction of Au(III). Previously, performed batch studies showed that Au(III) binding is fast, occurring within the first 5 min of contact and in a pH dependent manner. However, esterified hop biomass behaved in a pH independent manner and the binding was found not to change with changing pH. However, the hydrolyzed biomass had a similar Au(III) binding to the native hops biomass, showing a pH dependent binding trend. X-ray absorption spectroscopy (XAS) analysis, XANES (X-ray absorption near edge structure), and EXAFS (extended X-ray absorption fine structure) were used to determine the oxidation state, coordination environment, and the average radii of the gold nanoparticles bound to the hops biomass. The XAS data confirmed the presence of Au(0) in both the native and chemically modified hop biomasses. XANES fittings show that the Au(III) was reduced to Au(0) by approximately 81%, 70%, and 83% on the native, esterified, and hydrolyzed hop biomass, respectively. In addition, the calculation of the particle radius was also in agreement with the results of previously performed transmission electron microscopy studies. The average particle could only be calculated for the native and esterified hops biomass, which showed average particle radii of 17.3 2 and 9.2 2, respectively. D 2005 Elsevier B.V. All rights reserved. Keywords: X-ray absorption spectroscopy; Hop biomass; Gold nanoparticles; Metal binding

1. Introduction The importance of gold has been known since early times. In 1600, Paracelsus synthesized colloidal gold through the reduction of auric chloride with an alcoholic extract of plants that he called baurum potableQ. The red color in the resulting solution is characteristic of colloidal gold. Colloidal gold was used as a drug with medicinal and therapeutic properties [1]. Recently, the importance of gold in the jewelry industry and nanotechnology has increased due to the development of new technologies in electronics, chemistry, and engineering, all which work to design new materials with specific optical and electrical properties used in biosensors or molecular wires [2–4]. Several industrial processes such as gold electroplating and gold mining use cyanide and other related hazardous T Corresponding author. Tel.: +1 915 747 8678; fax: +1 915 747 5748. E-mail address: [email protected] (J.G. Parsons). 0026-265X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2005.01.011

compounds and the resultant effluents are very toxic to the environment. The effluents from these processes contain low gold concentrations in large volumes, which have considerable value. Therefore, there is an essential need to develop new recovery technologies that are both environmentally friendly and cost-effective [5]. Biological systems have the availability to accumulate specific metals from wastewaters. This process is called bioremediation. One type of bioremediation is phytofiltration, which uses dead tissues from plants to remove metal ions from water. Dead tissues from plants have been used to recover gold from aqueous solutions, more specifically, alfalfa (Medicago sativa) which has been studied for its ability to reduce Au(III) to Au(0) and to produce gold nanoparticles [6]. X-ray absorption spectroscopy has been extensively used in many fields to observe the solid-state phase transitions and the changes in valence and structure of materials. This analysis is very important in designing materials with

M.L. Lo´pez et al. / Microchemical Journal 81 (2005) 50–56

desired properties for industrial applications [7]. X-ray absorption spectroscopy is divided in two corresponding techniques: X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS). XANES is sensitive to the formal oxidation state and coordination chemistry of the absorbing atom, while EXAFS is more sensitive to interatomic distances, coordination number, and species of the neighbors of the absorbing atom [8]. Previous XAS studies have shown the reduction of Au(III) to Au(0) by exposing aqueous solutions of Au(III) to alfalfa biomass. In addition, EXAFS studies have suggested that Au(III) is first reduced to Au(I) before being finally reduced to Au(0), in which gold nanoparticles are then produced. XAS studies have also shown that gold binding to biomaterials may occur through nitrogen or oxygen ligands [6]. Transmission electron microscopy (TEM) has also been used to corroborate and characterize the size and the shape of gold nanoparticles formed in alfalfa biomass. In this study, the authors found gold nanoparticles with different shapes: face cubic centered (fcc), tetrahedral, hexagonal platelet, icosahedral multiple twinned, decahedral multiple twinned, and irregular shaped particles [9]. XAS has been used to study the metal binding of copper, zinc, and chromium to hop biomass in order to determine the oxidation state of these metals after they were reacted with hop biomass. These studies showed that only chromium(VI) was reduced by hop biomass to chromium(III), while copper(II) and zinc(II) bound to hop biomass with the same oxidation state. Also, these studies revealed that the nearest neighboring atom for all the metal ions was an oxygen atom [10]. Hop biomass has been studied for its ability to bind Au(III) ions from aqueous solutions (unpublished data). Different functional groups are implicated in the Au(III) binding mechanism and the ligands involved in the process have not been determined. The objective of this study is to corroborate the reduction of Au(III) to Au(0) from aqueous solutions by dead tissues of hop biomass through XAS and determine the nature of the chemical ligands involved in the Au(III) binding mechanism. X-ray absorption spectroscopy was used for both native and chemically modified hop biomasses reacted with aqueous solutions containing Au(III).

2. Materials and methods 2.1. Hop collection Hop biomass samples (cone belt) used in these studies were obtained through the USDA-ARS Hop Research Farm located in Corvallis, Oregon. Plants were grown until maturity and after flavonoids extraction, the cones were separated from the rest of the plant and washed with deionized water (DI) to remove dirt and debris. Samples were allowed to air dry for 1 week. Subsequently, the

51

samples were dried in a forced air dryer set at 65 8C and ground with a Wiley–Mill to pass through a 100-mesh sieve to achieve a uniform particle size. 2.2. Chemical modification of hop biomass Hop biomass was modified in order to determine the functional groups that could be involved in the binding of Au (III) to hop biomass and also to investigate the role of these groups on gold nanoparticle formation. Modification of hop biomass was carried out following procedures described in the literature by others [11]. 2.3. Chemical modification by esterification Carboxyl groups present in hop biomass were blocked by esterification. Nine grams of hop biomass was washed twice with 0.01 M HCl in order to remove any soluble material that may interfere with the Au(III) ions. Hop biomass was then washed three times with deionized water and centrifuged at 3000 rpm in a Marathon 6 K Fisher scientific centrifuge. Washings were collected in a beaker, heated to boiling, dried, and weighed to account for any loss of biomass. The biomass was re-suspended in an acidic methanol solution (methanol in 0.1 M HCl), stirred, and heated at 60 8C for 48 h, time at which the reaction was complete. The basic reaction is represented bellow: BiomassZCOOH þ HCl þ CH3 OHZ BiomassZCOOCH3 þH2 O After the reaction, the solution was centrifuged and the supernatants were discarded. The biomass was then washed again tree times with deionized water to remove excess acid, centrifuged, and lyophilized for further metal binding experiments. 2.4. Chemical modification by hydrolysis Hydrolysis of ester groups in hop biomass was carried out following the procedures previously described in order to increase the availability of carboxyl groups that may be involved in the binding of gold (III) [11]. The biomass was washed twice with 0.1 M HCl and three times with deionized water as described before. After that, the biomass was reacted with 100 ml of 0.1 M NaOH for 1 h in order to perform the following reaction: BiomassZCOOCH3 þ NaOHZ BiomassZCOO þCH3 OH þ Naþ After the reaction was carried out, the solution was centrifuged and the supernatants were discarded. The biomass was washed twice again with deionized water in order to remove any excess of sodium hydroxide, centrifuged, and lyophilized for further metal binding experiments.

52

M.L. Lo´pez et al. / Microchemical Journal 81 (2005) 50–56

2.5. X-ray absorption spectroscopic data collection A solution of 1000 ppm of Au(III) was prepared from KAuCl4 following the procedures described by others [10]. A 100 mg sample of native and modified hop biomass was weighed separately and washed twice with 0.01 M HCl to remove any debris. The samples were centrifuged in a Marathon 6 K Fisher scientific centrifuge and the supernatants were discharged. Three washings were completed with deionized water to remove the excess acid. Each biomass was adjusted to pH 4 and equilibrated with 20 mL of the 1000 ppm Au(III) solution at pH 4 for 1 h in order to saturate all the available binding sites. After equilibration, the samples were centrifuged and the biomasses were saved. The biomasses were frozen in liquid nitrogen for 45 min and then lyophilized using a Labconco Freeze Dry System. After lyophilization, the samples were mounted onto 1.0 mm sample plates with Mylar tape windows for XAS analysis at SSRL (Stanford Synchrotron Radiation Laboratories). The X-ray absorption spectra were measured on beam line 7–3 at SSRL. The samples were run in a liquid helium cryostat maintaining a temperature of 20 K to reduce the Debye–Waller effects. The data were collected using the Au LIII edge with energy of 11.918 keV using an Au foil internal standard to calibrate the edge energy. The beamline operating conditions were: an energy of 3 GeV, and a current ranging from 60–100 mA. The data were collected using a Si(220) double crystal monochromator with a B 90 orientation, a 1 mm of aperture, and with a resolution of approximately 1–2 eV. In addition, fluorescence spectra were collected from the samples, using a Canberra 30element germanium detector. Several scans were performed in order to improve the signal to noise ratio. The model compounds were diluted using boron nitride to give a one absorption unit change in the absorption across the absorption edge. The model compound dilution was performed by grinding the model compound with a mortar and pestle with a predetermined mass of boron nitride. 2.6. XAS data analyses In order to analyze the results obtained in this experiment, the Win-XAS software package was used and the data were extracted using standard methods [12,13]. The samples were calibrated using a 2nd degree derivative of the internal Au foil (LIII edge energy of 11.918 keV). Subsequent to energy calibration of the samples’ edge energies, a background correction was performed to extract the XANES region. The samples were background corrected using a one-degree polynomial fitting on the pre-edge region and the samples were normalized to make the edge jump equal to one absorption unit. The spectrum was then extracted from 11.86 to 12.05 keV. The same procedure was used on the 1 Am Au film and the model compounds, AuOH and KAuCl4. The XANES region was then characterized using

LC-XANES (linear combination XANES) fittings of the three model compounds. The EXAFS were extracted from the XAS spectrum using the standard methods [12,13]. The samples were converted into k space (or Wave Vector space) based on the energy of the photoelectrons ejected from the sample (calculated from the inflection point of the edge). The resulting spectra were then extracted using a cubic spline between 2.0 and 16.2 2 and k weighted to 3, and Fourier transformed in a modified Hanning window (over the first and last 10% of the spectrum). The samples were then back transformed and the first shell EXAFS were extracted and fitted using the ab initio multiple-scattering code FEFF V8.00 [13]. The crystallographic inputs for the FEFF fittings were created using atoms based on the crystallographic position of gold atoms in bulk gold metal. The model compounds were fitted using crystallographic data of their ideal crystal structures [14].

3. Results and discussion XAS was used to corroborate the reduction of Au(III) to Au (0) in native and chemically modified hop biomasses. LC-XANES analyses were performed on the XANES spectra to determine the amount of reduction that had occurred in the gold-biomass samples. LC-XANES fittings have been previously used to determine the composition of a number of different elements in environmental and materials science samples [15–18]. Fittings of samples using LCXANES provide information on both the oxidation state of the elements of interest and the coordination geometry of the element in the samples. For example, XANES fittings have been used to determine the oxidation state of sulfur in sulfur globules from bacteria, the composition of Mn in particles from car exhaust, and the composition of chromium in coal samples [15–17]. In addition, LC-XANES fittings have been used to determine the extent of chromium reduction in mesquite plants, which showed that mesquite plants grown on Cr(VI) contaminated media contained only chromium(III) inside the plant tissues [18]. Thus, LCXANES fittings are an invaluable tool to determine the oxidation state of an element, including gold in many different samples. In the determination of oxidation states for many of the lower mass elements, the oxidation state is based on the position of the edge or on pre-edge features. However, in dealing with the heavier elements such as Au, Pb, Pt, or U where the LIII edge is used to measure the absorption spectrum, the change in oxidation state is based on changes in the white lines. Using LIII edges, there are slight changes in the energy position of the adsorption edge, however, these are small, in comparison to the shifts in energy observed when investigating lower mass elements where the k edges are measured for the absorption spectra. In the case

M.L. Lo´pez et al. / Microchemical Journal 81 (2005) 50–56

Normalized Absorption

1.0 Hydrolyzed hop Native hop Esterified hop 1 µm Au film KAuCl4

0.75

0.5

0.25

Table 1 Percents of AuOH, Au(0), and KAuCl4 in native, esterified, and hydrolyzed hop biomasses at pH 4 Sample

%Au(I)OH

%KAu(III)Cl4

%Au(0)

Au native hop biomass Au hydrolyzed hop biomass Au esterified hop biomass

2.7 2.2 5.0

16.0 14.3 25.0

81.3 83.5 70.0

Fourier Transform Magnitude

These types of absorption spectra have been observed in many samples where gold is partially reduced such as the production of gold nanoparticles using alfalfa biomass [20]. The results of the LC-XANES fittings also confirm that the esterified hop biomass has the highest percentage of oxidized gold present at approximately 30% (Table 1), in a mixture of Au(III) and Au(I). On the other hand, the other biomass samples show around 15–20% of oxidized gold present (Table 1). Similar results for the reduction of Au(III) on alfalfa biomass have been observed; however, the reduction of Au(III) by alfalfa biomass required approximately 12 h to get a similar Au(0) spectrum [6]. Fig. 2 shows the Fourier transformed first shell EXAFS (k3) spectra for native, hydrolyzed, esterified hop biomasses, as well as the model compounds tetrachloroaurate, Au(I) hydroxide, and the gold foil. EXAFS fittings are not as precise for fitting of coordination numbers (N) as they are in fitting the interatomic distances. In dealing with EXAFS fittings, the generally accepted error on the fittings is approximately F20% of the obtained values, which

Fourier Transform Magnitude

of Au, the white line feature appears at approximately 11.918 keV and it changes drastically with changes in oxidation state as can be observed in Fig. 1 where the differences between Au(III) and Au(0) are shown. The white line feature present in Au(III), KAuCl4, is absent in the 1 um Au(0) spectra. The white line feature for gold is attributed to the transition of a 4p electron in to the 5d orbital, which manifests the energy change into a large increase in the absorption, this occurs in oxidized gold where there is an unfilled d orbital. However, in the case of Au(0), the d orbital has a d10 electron configuration or a filled orbital and no electrons can be added thus the white line is absent from this spectra [19]. Thus, the white line feature and the intensity of this feature can be diagnostic for the determination of the oxidation state of Au in a sample. In addition, the intensity of the white line can be fitted and used to determine the concentrations or atomic percentages of gold in a sample. Fig. 1 shows the XANES spectra of gold bound to the native, esterified, and hydrolyzed hop biomasses reacted with Au(III) at pH 4 compared with a 1 Am Au(0) film and KAuCl4. The absorption edge energies of Au(0) on native and chemically modified hop biomasses and the gold film model compound are, for the most part, identical (around 11.918 keV). In addition, the absence of the Au(III) white line feature at 11.918 keV shows that that the majority of the gold present in the samples is present as Au(0). The native and chemically modified hop biomasses also have an absorption maximum at 11.945 keV and 11.97 keV, which are characteristic of Au(0). These patterns corroborate that approximately all the Au(III) have been reduced to Au(0) after reaction with the native and chemically hydrolyzed hop biomass. Also, this figure displays the XANES spectra for KAuCl4 and we are able to detect a little shift in energy, compared with the Au(0) foil and the samples that corresponds to the Au(III) spectra. Furthermore, the Au(III) reacted with the esterified hops sample shows the presence of the gold white line feature and, although very small, it still shows the presence of a higher gold oxidation state than is observed in the other samples.

0.6

0.4 1 µm Gold Film Native hop biomass 0.2 Esterified hop biomass Hydrolyzed hop biomass

0.15

Tetrachloroaurate 0.1

0.05 Gold(I) Hydroxide

0 11.9

11.95

12

53

0.5

1

1.5

2

2.5

3

3.5

R [Å]

Energy [keV] Fig. 1. XANES spectra of Au bound to native, esterified, hydrolyzed hop biomasses at pH 4, and gold film.

Fig. 2. Fourier Transform first shell EXAFS spectra for native, esterified, and hydrolyzed hop biomasses at pH 4. Also, the EXAFS spectra for the gold film, gold(I) hydroxide, and tetrachloroaurate are shown.

54

M.L. Lo´pez et al. / Microchemical Journal 81 (2005) 50–56

corresponds usually to a Fone coordinating ligand in the sample. However, the interatomic distances calculated with EXAFS usually are within F0.01 or F0.005 of the true interatomic distance, and are dependent on how far into k space the sample was recorded. Thus, a combination of XANES and EXAFS fittings can provide the true structure of an unknown sample. These EXAFS spectra illustrate peaks showing the relative radial distances between the absorbing Au atom and the neighboring atoms (Fig. 2). The distances in angstroms are shown in Table 2, which also displays the coordination number and the Debye–Waller factor. The results of the fittings show that the nearest neighboring atoms in all the samples, for the most part, consist of gold atoms. However, the coordination numbers differ between each of the different modifications to the hops biomass and the interatomic distances for the Au atoms remain identical in all cases (2.89 2). In native and hydrolyzed hop biomasses, the interactions include additional Au atoms while in esterified hop biomass, we observe interactions with oxygen and chlorine atoms at 2.10 and 2.32 2, respectively. The structure in the EXAFS for the esterified hops biomass is significantly different from that observed in the EXAFS for the Au(0) foil and the gold reacted with the native and hydrolyzed hop biomasses. The esterified biomass shows a small shoulder at approximately 2.0 2 (not phase and amplitude corrected). However, the phase correction for this sample is approximately +D 0.35 2 which places this shoulder at approximately 2.35 2. The appearance of a Au–Cl bond occurs at approximately 2.32 2, which is very close in distance to the presence of the shoulder on the Au(0) peak. This indicates that Cl is bound to some of the gold on the biomass. In addition, the esterified biomass shows a small peak at approximately 1.6 to 1.7 2, this indicates that there is some oxygen/nitrogen bound to the gold in addition to the presence of the Cl ligand. The fitting of mixed oxidation state EXAFS has not been successfully done and in this case, it is used to identify the potential ligands present that are bound to the gold atoms on the sample. The similarity of the spectra for the

Table 2 Interatomic distances, coordination numbers, and Debye–Waller factor in native, esterified, and hydrolyzed hop biomasses at pH 4, as well as in the gold foil and gold model compounds Sample

Bond

N

R (2)

r 2 (22)

Radius

Native hop biomass Esterified hop biomass

Au–Au Au–Au Au–Cl Au–O Au–Au

10.5 9.2 0.8 0.9 11.1

2.89 2.89 2.32 2.10 2.89

0.0023 0.0022 0.0018 0.0010 0.0024

17.3 9.21 N/A N/A N/A

Au–Au Au–O Au–Cl

12.0 1.0 4.0

2.88 1.97 2.29

0.0030 0.000019 0.0015

N/A N/A N/A

Hydrolyzed hop biomass 1 Am gold foil Gold(I) hydroxide Tetrachloroaurate

N represents the coordination number, R is the interatomic distance in 2, and r 2 is the Debye–Waller parameter given in 22.

native and chemically hydrolyzed hop biomasses indicates that the binding mechanism for Au(III) to hop biomasses is similar; however, further analysis is needed to determine the atoms that are involved in this binding mechanism. Table 1 shows Au(0)% in native and chemically modified hop biomasses. Approximately 80% of Au found in native and hydrolyzed hop biomasses is present as Au(0) and the other 20% correspond to a possible mixture of the KAuCl4 and AuOH present in the samples. It has been shown that the size of nanoparticles of FCC metals can be calculated from the EXAFS fittings of the first shell. Borowosky first showed it in estimating the size of copper nanoparticles and later Gardea-Torresdey et al. determined the size of gold nanoparticles [20]. The equation used to calculate the size of the nanoparticles is as follows [21]: 3 1 Fr ¼ R3 ðNratio  1Þ þ dr2  d 3 4 16 The parameters in the equation include the following: r is the average radius of the nanoparticles, d is the interatomic distance between the neighboring atoms, and N ratio is the ratio of the number of neighboring atoms in the nanoparticle divided by the number of neighboring atoms in the bulk metal [19,21]. One drawback in this equation is that it assumes a spherical particle, which may not be absolutely true for gold nanoparticles. This equation accounts for changes not only in the number of nearest neighboring atoms but also changes in interatomic distances, which have been observed in some FCC metals like silver, where the interatomic distances shrink with changes in particle size [22]. From calculations based on this equation, the average radius of the hop produced gold nanoparticles was calculated to have been 17 2 and 9 2 for the native and esterified biomass, respectively. However, the hydrolyzed hop biomass has an average particle size above 25 2, which is too large to be calculated using EXAFS and the aforementioned equation. The average particle size translates into particles with an average diameter between 3.0 and 4.0 Dm for the native hop biomass and approximately 2.0 Dm for the esterified hop biomass. These calculations are in good agreement with previously obtained transmission electron microscopy results on the same system (unpublished data). In addition, these results of the calculated sizes of the gold nanoparticles produced by hop biomasses are similar to the sizes of Au(0) nanoparticles calculated produced using inactivated cells of alfalfa biomass [20]. Alfalfa biomass produced gold nanoparticles with radii between 6.2 2 and 9 2 for native alfalfa at pH 5 and pH 2, respectively [20]. EXAFS spectroscopy calculations of the radii of the gold nanoparticles produced by the native hops biomass show that the average size is almost double that calculated for native alfalfa biomass. This difference in size may be attributed to the contact time of the gold with the biomass, the hops reactions may have been longer and thus produced

M.L. Lo´pez et al. / Microchemical Journal 81 (2005) 50–56

large particles. On the other hand, it may also be due to the differences in the number and types of functional groups on the hops biomass. The cause of the formation of different size nanoparticles between the two biomasses needs to be investigated further. However, the calculated sizes of the nanoparticles created by the esterified hops biomass are in the same size range of those previously produced using alfalfa biomass, which are smaller than the nanoparticles produced by both the native and hydrolyzed hops biomass. The difference in the size of the nanoparticle sizes appears to be linked to the availability of the chemical functional groups on the biomass, specifically the carboxyl groups. The difference in the sizes may then be linked to gold chemistry in aqueous solutions. In aqueous solution KAuCl4 undergoes a number of ligand exchange reactions with hydroxyl ligands in water, the reactions are shown below [23]:    AuCl 4 þ OH pYAuCl3 ðOHÞ þ Cl  AuCl3 ðOHÞ þ OH pYAuCl2 ðOHÞ 2 þ Cl    AuCl2 ðOHÞ 2 þ OH pYAuClðOHÞ3 þ Cl    AuClðOHÞ 3 þ OH pYAuðOHÞ4 þ Cl

These series of reactions may be what is occurring on the biomass to form a carboxyl–gold bond, initially binding the gold to the biomass for the electron transfers to occur between the gold and the biomass, where the AuClx (OOR)X species may be more receptive to the electron transfers than in solution. Thus, a higher number of carboxyl groups on the biomass that are closer together may result in a higher number of gold nanoparticles produced that are larger in their average radius, due to their proximity. The closeness of the carboxyl groups may allow the coalescence of smaller particles to occur and form larger particles if the carboxyl groups are close, as is observed with the native and hydrolyzed hop biomass. In addition, this idea is supported as hot citric acid, a carboxylic acid is used to reduce aqueous Au(III) ions into Au(0) nanoparticles [24]. This reduction indicates that the carboxyl groups play an important role in the mechanism of Au(III) reduction to Au(0). However, the role of the carboxylic acids in the reduction mechanism needs to be studied further.

4. Conclusions XAS studies performed on native and chemically modified hop biomasses have shown a significant result: the ability to bind Au(III) and reduce it to Au(0), and these studies may have indicated the initial steps of the mechanism of gold(III) reduction by hop biomass. The XANES spectra corroborated the reduction of Au(III) to

55

Au(0) for both the modified and native hop biomasses. However, the extent of Au(III) reduction is different for the hydrolyzed and esterified hop biomasses. Our results indicate that the binding mechanism for Au(III) is similar for both native and hydrolyzed hop biomasses. However, esterified hop biomass suggested that Au(III) ions bind to the biomass in a different type of ligand and also the esterification process reduces the efficiency of the reduction process. EXAFS spectra showed that the nearest neighboring atoms were Au atoms in native and hydrolyzed hop biomasses. However, the esterified hop biomass showed that Au neighbors were present, but there were also oxygen or nitrogen, and chlorine ligands attached to the gold. The results of the esterified biomass show the importance of the carboxyl groups on the biomass in the gold(III) reduction to gold(0). Potentially, the carboxyl or other oxygen containing ligands on the biomass are the groups responsible for the binding and the reduction site for gold on the biomass. The importance of the carboxyl groups in the reduction of gold by the biomass is supported by the chemical production of gold nanoparticles using citrate in aqueous solutions. Further experiments are needed in order to determine the atoms in the functional groups that are involved in the binding of Au(III) to hop biomass. On the other hand, esterified hop biomass produced gold nanoparticles with a smaller radius than the native and hydrolyzed hop biomasses.

Acknowledgments The authors would like to acknowledge the National Institutes of Health (grant S06 GM8012-33) and the University of Texas at El Paso’s Center for Environmental Resource Management (CERM) through funding from the Office of Exploratory Research of the U.S. Environmental Protection Agency (cooperative agreement CR-819849-01). We also thank the financial support from the Southwest Center for Environmental Research and Policy (SCERP) program, and the HBCU/MI, Environmental Technology Consortium that is funded by the Department of Energy. Dr. Gardea-Torresdey acknowledges the funding from the National Institute of Environmental Health Sciences (Grant R01ES11367-01) and the Dudley family for the Endowed Research Professorship in Chemistry. Portion of this research was carried out at the Stanford Synchrotron Radiation Laboratory (SSRL), a national user facility operated by Stanford University on behalf of the US Department of Energy (DOE), Office of Basic Energy Sciences. Martha L. Lopez also acknowledges the Consejo Nacional de Ciencia y Tecnologia of Mexico (CONACyT) for its financial support (Grant 178763). Dr. Gardea-Torresdey also acknowledges Dr. John Henning for providing hop biomass utilized in this study.

56

M.L. Lo´pez et al. / Microchemical Journal 81 (2005) 50–56

References [1] C. Gasparini, Gold and Other Precious Metals, Springer-Verlag, Berlin, 1993, p. 2. [2] M. Brust, C. Kiely, Colloids Surf., A Physicochem. Eng. Asp. 202 (2002) 175. [3] A. Lazarides, K.L. Kelly, T.R. Jensen, G.C. Schatz, J. Mol. Struct., Teochem 529 (2000) 59. [4] Ch. Lee, Y. Kang, K. Lee, S.R. Kim, D. Won, J.S. Noh, H.K. Shin, C.K. Song, Y.S. Kwon, H. So, J. Kim, Curr. Appl. Phys. 2 (2002) 39. [5] A.V. Pethkar, K.M. Paknikar, J. Biotechnol. 63 (1998) 121. [6] J.L. Gardea-Torresdey, K.J. Tiemann, J.G. Parsons, G. Gamez, I. Herrera, M. Jose-Yacaman, Microchem. J. 71 (2002) 193. [7] L.C. Feldman, J.W. Mayer, Fundamentals of Surface and Thin Films, Elsevier Science Publishing Co. Inc., 1986, p. 233. [8] A. Jentys, L. Simon, J.A. Lercher, J. Phys. Chem. 104 (2000) 9411. [9] J.L. Gardea_torresdey, K.J. Tiemann, G. Gamez, K. Dokken, S. Tehuacanero, M. Jose-Yacaman, J. Nanopart. Res. 1 (1999) 397. [10] J.G. Parsons, M. Hejazi, K.J. Tiemann, J. Henning, J.L. GardeaTorresdey, Microchem. J. 71 (2002) 211. [11] J.L. Gardea-Torresdey, M.K. Becker-Hapak, J.M. Hosea, D.W. Darnall, Environ. Sci. Technol. 24 (9) (1990) 1372. [12] T. Ressler, J. Synchrotron Radiat. 5 (1998) 118.

[13] T. Ressler, J. Phys., IV 7 (1997) C2. [14] B. Ravel, J. Synchrotron Radiat. 1 (2001) 314. [15] I.J. Pickering, G.N. George, E.Y. Yu, D.C. Brune, C. Tuschak, J. Overmann, T.J. Beatty, R.C. Prince, Biochemistry 40 (27) (2001) 8138. [16] J.G. Reynolds, A.J. Nelson, J. Wong, J. Roos, Book of Abstracts, 219th ACS National Meeting, San Francisco, CA, March 26–30, 2000. [17] G.P. Huffman, F.E. Huggins, N. Shah, J. Zhao, Fuel Proces. Technol. 39 (1–3) (1994) 47. [18] M.V. Aldrich, J.L. Gardea-Torresdey, J.R. Peralta-Videa, J.G. Parsons, Environ. Sci. Technol. 37 (9) (2003) 1859. [19] J.G. Parsons, M.V. Aldrich, J.L. Gardea-Torresdey, Appl. Spectrosc. Rev. 37 (2002) 187. [20] J.L. Gardea-Torresdey, K.J. Tiemann, G. Gamez, K. Dokken, I. CanoAguilera, L.R. Furenlid, M.W. Renner, Environ. Sci. Technol. 34 (2000) 4392. [21] M. Borowski, J. Phys., IV 7 (1997) C259. [22] P.A. Montano, J. Zhao, M. Ramanathan, G.K. Shenoy, W. Schulze, J. Chem. Phys. Lett. 164 (2) (1989) 126. [23] J.A. Peck, C.D. Tait, B.I. Swanson Jr., G.E. Brown, Geochim. Cosmochim. Acta 55 (3) (1991) 671. [24] E. Elliott, C. Dennison, Anal. Biochem. 186 (1) (1990) 53.